docs: def require_qt(func):
      """Specify that a function requires a Qt application.
  
      Use this decorator to specify that a function needs a running
      Qt application before it can run. An error is raised if that is not
      the case.
  
      """
      @wraps(func)
      def wrapped(*args, **kwargs):
          if not QApplication.instance():  # pragma: no cover
              raise RuntimeError("A Qt application must be created.")
          return func(*args, **kwargs)
      return wrapped